This invention relates to engine combustion processes and in particular to a dual mode combustion process involving combustion of flame extinguishing dilute fuel-air mixtures at lower engine speeds and loads and flame propagating combustion fuel-air mixtures at higher speeds and loads.
It is known that high fuel economy and low NOx emissions can be achieved in internal combustion (IC) engines operating with unthrottled dilute combustion. Dilution of the charge may be achieved with excess air and/or recirculated exhaust gas. Homogenous Charge Compression Ignition (HCCI) combustion has been demonstrated as one means to achieve combustion in ultra dilute fuel-air mixtures. By ultra dilute mixtures is meant those mixtures with dilution levels beyond that which will support propagating turbulent flame combustion, that is, flame extinguishing mixtures as compared to flame propagating mixtures which will support propagating turbulent flame combustion. HCCI combustion has been considered impractical because it can only be achieved over a limited range of engine speeds and loads and is difficult to control under transient operation. A major cause of these limitations is the fact that charge ignition timing is controlled by the temperature-time history and fuel-air kinetics of the premixed charge. A second limitation is that HCCI cannot be employed at high loads due to diminishing charge dilution with increasing load which leads to excessive heat release rates, high peak cylinder pressures and combustion induced noise.
The present invention overcomes the limitations of the known HCCI combustion process by employing a dual mode combustion process. For light loads, a first combustion mode is employed utilizing Premixed Charged Forced Autoignition (PCFA) for igniting an ultra dilute premixed charge in a combustion chamber. In this mode, so called Pulse Jet Ignition (PJI) is used to force a spark ignited jet of hot reacting fuel mixture from a precombustion chamber, or prechamber, into the ultra dilute premixed charge compressed in the main combustion chamber of the engine. The jet of burning gases mixes with part of the main combustion chamber charge causing rapid combustion of this fraction of the charge, and the cylinder pressures are raised thereby until the remainder of the ultra dilute mixture is ignited by compression ignition. Burning of the ultra dilute mixture in this manner allows control of ignition timing to the proper point in the cycle and the resulting combustion of the ultra dilute mixture in the cylinder provides combustion with reduced combustion temperatures and low NOx emissions.
For higher speeds and loads, a second combustion mode is utilized wherein the main combustion chamber is charged with a less dilute fuel-air mixture which supports flame propagation when ignited. The flame propagating combustible fuel-air mixture is compressed in the combustion chamber and ignited conventionally with spark ignition or, alternatively if desired, with pulse jet ignition. In either case, combustion progresses in a normal flame propagation mode, eliminating the problems associated with HCCI combustion at the higher speeds and loads. Thus, the combination of combustion modes provides a dual mode engine combustion process which allows use of ultra dilute mixtures where conditions permit providing reduced emissions and improved economy, combined with conventional propagating turbulent flame combustion at higher loads and speeds.
In a preferred embodiment of the dual mode process of the invention, each cylinder of the engine is provided with a twin-spark gap pulse jet igniter (PJI). The igniter is formed similar to a spark plug but includes an internal prechamber opening to the main combustion chamber of the engine. The igniter includes a fuel supply means for supplying small amounts of fuel to the prechamber. It further includes an electrode system involving dual spark gaps including a first spark gap within the internal prechamber and a second spark gap adjacent the outlet of the prechamber for ignition of mixtures within the main combustion chamber.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.